Rocket mass heater and length of under-bed pipe

Hi, I am new to the idea of rocket mass heaters and to Permies.com, and I am going to build a hoop house. We are going to set up aquaponics and as the winters can be quite long and cold, I wondered just how long the under-bed pipes can be. The hoop house will be 10 meters by 5 meters. Can I run the pipes around the sides all around the hoop house and get heat to the whole area? Hope you can help. Thanks

Gordy-
Welcome to the permies forum. If you are able you need to get the new 3rd edition of the rocket stove heaters book here RocketStoves.com
It will give you a lot of info about designing a RMH.
As far as a hoop house goes of the given dimensions, an 8" RMH system might work. It's maximum exhaust duct length is up to maybe 50ft or~16m.
There are a lot of other factors involved in designing a hoop house with a RMH for heat.
Using the formula A/R x deltaT=BTU/Hr: A= surface area of structure, R=insulation "R"factor (typical 6mil plastic cover is R=1) Delta T is the difference between outside temp and desired inside temp (deg F)) This will tell you how much heat you need to heat a given space.
A quick example using your dimensions covered with 6mil plastic- approx area=900sqft /R of 1=900 x 70 (0deg outside 70deg inside) =63,000BTU's/hr. This might be a low temp, but worst case is a place to start. A typical 8"RMH might produce 75k BTU (this can vary considerably) so you might have a bit of a problem storing enough excess heat beyond the immediate needs to maintain 70deg F.
First suggestion I would give is to have 1 long side facing south and more heavily insulate the north side and the ends. Straw bales as an inexpensive example.
Second don't bury the RMH duct in the ground unless you're prepared to do a little more work isolating/insulating it from the surrounding earth. Having the mass above ground and under the grow beds is a good option. Also consider using water for the thermal mass (besides the AP fish tank).
The grow beds and fish tank will help in maintaining the heat, but they will also need to be heated too.
Quite a bit to think on so let the questions fly

Hi John, thank you for such an in-depth reply. As you said, there is a lot to think about. Several points you made hit the spot right away, especially about the north facing long side and having the mass above ground. I have been looking into using water as an added part to the thermal mass, and you are on the button as I would be reaching to maintain the 72deg that I would need and maintain it. I am going to study all that you have said, and go for the book as you suggested. Gosh, and I thought it looked fairly easy! Well, I am going to get into this and will come back to you as things progress, and especially if I run into something that I can't fathom. I had a thought during the night that I could perhaps shorten the run by going down the middle instead of the sides, and having the grow beds in the middle partially over the mass at both sides, with the fish tank at one end and with the heater at the other?. I didn't sleep long as you can imagine. I am going to plan this well, and with your help and the information from the book, I should be well armed. Thank you so much John, I am sure to be back, but your equations really jump out at me and help a lot.

As far as the calculations for heating needs it assumes an empty space. Like I said it's a starting point. All the mass of grow beds, fish tanks etc. kind of change things. When you have a fish tank at 70+deg in a cold weather situation you really need a way to maintain that separate from the RMH heating the mass. Heating the tanks from the RMH is a tricky proposition, and I've yet to see a simple foolproof system. Using separate water tanks for thermal mass is actually better than cob, but not many people are doing it.
Check out this video Death of the rocket stove as well as some of the other from Van Powell for Ideas. I got the formulas and info about BTU's from him.

Gordon Mitchell : You will also Enjoy the series of videos Fellow Member Rob Torcellini has posted over in you tube land just type web4deb in the U-tube Search Field !

For the Craft ! Think like Fire! Flow like Gas! Don't be the Marshmallow! As Always your comments and questions are Welcome and solicited ! PYRO - Logically ! Big AL